testsuite: arm: use effective-target for mod* tests

This fixes a typo introduced in r15-4200-gcf08dd297ca that was reported
at https://linaro.atlassian.net/browse/GNU-1369

.

gcc/testsuite/ChangeLog

	* gcc.target/arm/mod_2.c: Corrected effective-target to
	arm_cpu_cortex_a57_ok.
	* gcc.target/arm/mod_256.c: Likewise.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>